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A Glimpse into Cincinnati’s Fossil History

By: Olivia Eads

Have you ever noticed the frequency of shells and other organisms fossilized in limestone around the Cincinnati area? They’re everywhere—used as the framework in buildings, exposed on cliff faces, and in back yards—but where did they come from? In order to dive into this topic further we must take a trip back, deep into time.

Roughly 490 million years ago, Cincinnati was a completely different ecosystem in a very different geographical location. In geologic time, it was considered the Ordovician Period, a very dynamic period in biodiversity of organisms, glaciation, active and passive tectonic margins, and solar system cyclicity. During this period Cincinnati was located just south of the equator experiencing a shallow marine habitat and tropical weather conditions, a very similar ecosystem to that of coral reefs in the equatorial zones today. The extent of this bionetwork stretched through regions of Ohio, Kentucky, and Indiana, all considered part of the Cincinnatian Arch. The depths of water ranged, depending on geography, and were determined using context clues such as: fossilized organisms associated with living ancestors, rock types, and features present. The clues indicated an intertidal (above water at low tide, covered at high tide) to subtidal (submerged except during full/ new moon events) zones in Cincinnati.

When identifying lifestyles of the organisms that dwelled upon this area there are a few things to remember. Fossilization is a very intricate process and favors hard bodied organisms. Not all creatures or parts of their bodies are preserved. The Ordovician began, following the Cambrian explosion, a rapid burst of biodiversity in flora and fauna. It was a very dynamic time in both landscape evolution and species biodiversity. There are many things still unknown about the livelihood and behaviors of these organisms. A majority of that knowledge stems from what is already known about living fossils or the modern example of these ancestral species. Now to the fossils!

Bryozoans are considered ‘moss- like’ invertebrates. They typically grew in colonies attached to hard surfaces, but could also establish individually. Brackish and coastal waters are favored by these organisms due to the decreased salinity. Lophophores, tiny tentacle like structures, were used during feeding by popping out to collect tiny, suspended sediments, then bring them back to the mouth for digestion. The monticules or bumps along the surface of their bodies are suggested to be an escape current produced by the action of lophophores while feeding. The framework of these colonies created a large habitat for all types of aquatic life. Many of the fossils preserved are in pieces due to the ocean’s mechanical weathering on the skeletons.

Crinoids

These are another fossil that are typically found in pieces. The fragmented parts of the stem resemble small to large buttons or, if there are multiple larger ones stuck together, a roll of mints. A more common name for this animal is a sea lily. Their body cavities are divided into three main sections. The column or stem which consists of disc shaped endoskeleton stacked upon each other that are held together with ligaments. The calyx sits on top of the stem and holds the body cavity. Then there are the feathery arms that protrude out of the calyx. They collect suspended sediments in the water and bring it towards the mouth for digestion. Due to the soft and fragile nature of the arms, they are not typically preserved in the fossil record. The size of these organisms can range up to a few meters in length and typically attach to hard substrate with a holdfast; however, some modern species have been observed moving independently across the sea floor. The independent movement was more than likely an adaptation to the evolution of predators and probably not present in Ordovician sea crinoids.

Brachiopods are the most abundant shell fossil found in the Cincinnati area. They are divided into two main categories: articulated and inarticulated (based on the presence or absence of hinge teeth and sockets.) Both shells are symmetrical across the mid line, but the top and bottom shell are not equal in size. They have a pedicle or fleshy stalk that helps them attach to the sea floor or burrow down into sediments. As a filter feeder, they open and close their shell with currents allowing water and sediment to pass through. Using their lophophore, sediment is caught for consumption.

Bivalves

Bivalves include shelled organisms such as: clams, oysters, mussels and scallops. They were not very abundant during the Ordovician probably due their niche being previously occupied by brachiopods. The shells are symmetrical across the hinge line, and some have growth lines that can be observed. Some bivalve species permanently cement themselves to hard substrate, while others use their muscular tongue or foot to burrow down into the sediment. A siphon is then used for filter feeding to suck water into the shell cavity for subsistence.

Rugose Corals

Commonly known as horn corals, these organisms typically lived in a colony at the bottom of the ocean floor. As a colony, they created large reef like structures. However, some lived in solidarity. These microcarnivores had small tentacles that were used to catch prey and are bilaterally symmetrical.

Gastropods

Otherwise known as SNAILS! These mollusks eat anything and everything (herbivore, carnivore, omnivore, and scavenger.) During the Ordovician these creatures were marine dwelling and used a muscular foot to transport themselves around. The shell located on their back is for protection against predation and to house organs. Typically, their shells are the only thing preserved in fossils and they vary greatly in size and shape. All shells follow the same general spiral pattern. As time progressed snails were able to adapt to marine, aquatic, and terrestrial landscapes.

Trilobites

Trilobites are a very diverse group of organisms with over 20,000 different species with very different modes of life. Although now extinct their reign in the ocean lasted around three billion years, wide spread reaching every continent. Trilobites were some of the first organisms to have complex eye structures. Their body can be divided into three main segments: the cephalon (head), thorax (body), and pygidium (tail). Thanks to incredibly preserved fossils (such as the Burgess Shale), trilobites are shown to have soft appendages such as jointed legs and antennae. As they grew their exoskeleton did not grow with them. Instead they molted a chitinous skeleton (similar to a lobster’s) which is mainly preserved. In order to escape predation, they could roll up into a ball so that their exoskeleton was only exposed or burrow down into sediments. Different species took on very different modes of life.

Nautiloids

Nautiloids are cephalopods (in the same family as octopuses and squids) with many tentacles and straight shells. The shell has many chambers and a siphuncle in order to control buoyancy while swimming in their marine ecosystem. As they grow their shell secretes more material growing with them. They were the fearsome predator of the sea.

There are many more fossilized organisms and features in Cincinnati’s strata, but that dives a little too deep for the topics being discussed today. These organisms cover the basics of what are copious in Cincinnati’s fossiliferous limestone. There are quite a few fossilized features that represent storm events and the abundance and amount of weathering can give insight into environments where those fossils were deposited! Alas, those shall be saved for another day. This should be a good start in the exploration to Cincinnati’s geologic history. Hopefully, in the near future, some of you can join me in person on upcoming RRT sponsored hikes! We can discuss these features and fossils more in depth while learning identification techniques in the field! Until next time.